Most portable sawmills use band saws for bi-directional sawing. While band saws have a thinner kerf, it is hard to keep the band blade tracking a straight line through a round log, without dipping or diving. Band blades tend to produce a wavy cut. Sometimes an entire board or blank is wasted due to in accurate dimensions. Band saws are high maintenance and do not perform well in conditions in which the wood has been contaminated due to harvesting methods or environmental conditions. Circular saws lessen these problems, but do not lend themselves to bi-directional sawing. Traditional mills that use circular saws cut in one direction only, so that the return cycle is wasted time.